I am all out of suggestions - if the fuse is good and there's a call for heat and the boiler is on (has power) and the overtemp has not tripped and the boiler stat is not on zero and you still have no 230V between 7 & 8 then the only thing I can think of is a wiring fault - when you changed the control board you may have failed to connect something correctly - it does happen.
Perhaps it's time for an expert - not a well intentioned amateur like me...
Continuing good luck...
P
November 2009